How should client-side and server-side form validation work together?

Use client-side validation for responsive feedback and server-side validation as the authoritative check. Share clearly defined field rules where practical, but still validate request types, lengths, allowed values, and cross-field business rules on the server. Return structured field errors and preserve safe user input so users can correct mistakes. Database constraints provide another layer for invariants such as uniqueness. Consider race conditions: a username can become unavailable after a browser check. Validation errors should explain the correction without exposing internal stack traces or sensitive records.